Electro-optically generated displays, e.g., of the type used for virtual reality displays, video camera displays, computer displays, television displays and the like, can be generated by a number of image generators, including liquid crystal displays (LCDs), light emitting diode (LED arrays), cathode ray tubes (CRTs), plasma discharge devices, electroluminescent display, and micromirror displays. Many displays produce an image composed of a multiplicity of picture elements ("pixels"). In at least some of these devices, there is a space between adjacent pixels such that each pixels is surrounded by what is perceived by the user as a dark area. Pixelated displays in which there is a space between adjacent pixels has been found to be less satisfactory for most users than a display which has been depixelated. A process of depixelation involves filling-in the area between adjacent pixels, preferably with an intensity and/or color similar to the closest pixel.
Another problem which arises in pixelated displays is found in spatially color-distributed color displays. In this type of display, each pixel is made up of three sub-pixels, each displaying one of a set of three colors (e.g., red, green, blue), and each subpixel being separated from the others in a spatial fashion. It is desired that the user should perceive a single color at each pixel, and thus for this type of display it is desirable that depixelation among each set of subpixels should be sufficient to blend the colors together, such that, for example, if the image generator outputs substantially equally-intense red, green and blue subpixels, the viewer will perceive a single white pixel at that location.
Although the advantages of depixelation, in general, as described above, are appreciated, it has been difficult to achieve practical depixelation for a number of reasons. First, depixelation often involves blurring the pixels or the edges of the pixel. However, this must be done without losing resolution or contrast of the image to an unacceptable degree. Furthermore, some depixelation schemes are expensive or difficult to design, manufacture, repair, and/or maintain, and some depixelation schemes result in undesirable optical artifacts or side effects.
One depixelation scheme is described in U.S. Pat. No. 5,303,085 issued Apr. 12, 1994, to Rallison. Depixelation of, e.g. a liquid crystal display is provided through placement of a fiber optic base plate between the liquid crystal display and a beam splitter (a fold mirror, in this case), where the base plate has a numerical aperture related to pixel size and distance. Although this device appears to be effective for the purpose, it is desirable to also provide alternative depixelation, e.g., for purposes of supply, manufacturability, economy and the like.
Another process for depixelation with a fiber optic base plate involves placing the light source for the LCD near the rear of the pixel plane, such that the image of each pixel is enlarged, and also covers a greater portion of the total picture, correspondingly decreasing the amount of dark area surrounding each pixel. Also, the small size of each pixel causes considerable diffraction of the light which passes the edge of the pixel. Further, when a fiberoptic face plate is utilized, a diffuse source of light cannot be employed because it would create an image of each pixel on the input surface of the face plate, which would be excessively large. This forces the use of an incandescent lamp, which produces more heat which can negatively affect liquid crystal displays.
Another technique for depixelation involves positioning a weak diffuser plate a short distance from the pixel in the direction, e.g., of the bean splitter. A diffuser plate is a plate which disperses an incoming ray over a solid angle. Most diffuser plates are produced by a process of coating (e.g. painting) or mechanical (e.g. grinding or sandblasting) or chemical (e.g. etching) texturing of a surface, such as a reflecting surface or a surface of a transparent material. The diffusion produced by a weak diffuser plate is, however, in random directions, and usually of lambertian energy distribution. The weak diffuser has the disadvantage, from the point of view of a viewer, that most of the fight remains scattered and the image remains bright. These techniques do not control the modulation depth and diffractive angle well for narrow beam diffusion of small diameter input beams (such as about 20 microns).
Another approach to depixelation is using a crossed diffraction grating which is regular or periodic, typically having square or rectangular symmetry. One type of depixelator using controlled diffraction is described in PCT/US94/01390, for Depixelated Visual Display of Richard Rallison, filed on Feb. 7, 1994, and incorporated herein by reference. The depixelation effect achieved by a crossed grating can be adjusted by adjusting, e.g., the spatial frequency (inverse of the pitch) or the efficiency or modulation depth of the grating, and the material of the grating (e.g., as a volume holographic grating or as a surface grating). Crossed diffraction gratings, however (and, it is believed, all periodic depixelation structures) produce an undesirable moire pattern (also known as aliasing). The patterns may be changed or reduced by judicious use of grating frequencies and/or angles of crossing, but are difficult to reduce to the point where there is no substantial effect on viewing quality.
Accordingly, it would be useful to provide a depixelation device which is relatively inexpensive to design, manufacture, repair, and maintain, and which is substantially free from moire patterns or other undesirable artifacts or side effects.